Method of making molds

ABSTRACT

A method of making molds for use in casting highly reactive metals such as titanium, zirconium, hafnium, columbium, and alloys thereof in which granular graphite, olivine or mixtures thereof are mixed with a binder such as sodium silicate to form a mixture which is rammed into a flask about a pattern to form a mold. The mold is removed from the pattern with or without first hardening and is then baked at a temperature up to about 290* F. The mold is then coated with a thermally insulative coating which is nonreactive with the molten metal such as alumina in an acrylic binder. The mold is then ready for use or it can be stored indefinitely under conditions which prevent its picking up moisture.

Turner, Jr. et al.

[ METHOD OF MAKING MOLDS [75] Inventors: Dwight L. Turner, Jr.,'SantaAna;

Warren E. Lee, Upland, both of Calif.

[73] Assignee: Ti Tech International, Inc., Pomona,

Calif.

[22] Filed: Mar. 17, 1972 [211 App]. No.: 235,660

[52] US. Cl 106/38.3, 106/389, 106/3822 [51] Int. C1 B28b 7/34 [58]Field of Search 106/383. 38.35, 38.9

[56] References Cited UNITED STATES PATENTS 1,889,007 11/1932 Wallace106/383 2,892,227 6/1959 Operhall 106/3835 2,952,553 9/1960 llenda etal. 106/3835 1 Apr. 9, 1974 3,146,113 8/1964 Middleton et al. 106/3823Primary Examiner-Lorenzo B. Hayes Attorney, Agent, or FirmEdgar N. Jay

[ 5 7 ABSTRACT A method of making molds for use in casting highlyreactive metals such as titanium, zirconium, hafnium, columbium, andalloys thereof in which granular graphite, olivine or mixtures thereofare mixed with a binder such as sodium silicate to form a mixture whichis rammed into a flask about a pattern to form a mold. The mold isremoved from the pattern with or without first hardening and is thenbaked at a temperature up to about 290 F. The mold is then coated with athermally insulative coating which is nonreactive with the molten metalsuch as alumina in an acrylic binder. The mold is then ready for use orit can be stored indefinitely under conditions which prevent its pickingup moisture.

21 Claims, No Drawings METHOD OF MAKING MOLDS This invention relates tothe making of molds for use in foundry casting and, more particularly,to the making of such molds suitable for use in the casting of highlyreactive metals such as titanium, zirconium, hafnium, columbium andalloys containing such-reactive metals.

Hitherto, graphite molds have been used in the casting of such reactivemetals as titanium and its alloys, but such molds have left much to bedesired. Molds machined from graphite have proven to be expensive, evenfor casting relatively simple shapes, and not at all suited for thecasting of the more complex shapes required today. To overcome this,graphite in granular form mixed with a suitable binder is forced, as byramming, into intimate contact with a suitable pattern and fired atelevated temperatures above about lOOO F. Such firing not only addssignificantly to the cost of the molds, but also can result in shrinkageand distortion which interferes with the high degree of precisionusually required.

The use of molds as hitherto provided, whether formed by machining fromgraphite blocks or from granular graphite with a binder has alsoresulted in surface defects in the cast articles such as flowlines andnegative defects which have also added to the cost of the castings.

It is therefore a principal object of this-invention to provide animproved method of making foundry molds especially well suited for usein casting reactive metals such as titanium, zirconium, hafnium,columbium and alloys thereof having more or less complex shapes, whichpermits the use of conventional foundry techniques and equipment, whichprovides molds capable of enhanced precision and freedom from defects inthe parts cast therein, and that can be readily separated from thepatterns about which they are formed, and which method is relativelysimple andrequires less time to carry out.

Further objects as well as advantages of the present invention will beapparent from the following detailed description of preferredembodiments thereof. In carrying out the present invention, a suitablerefractory material such as granular graphite, a suitable sand such asolivine, or mixtures thereof are mixed with a suitable binder to providea mixture having a consistency suitable for ramming. A pattern is placedinto a flask, and the prepared refractory and binder mixture is rammedinto the flask about the pattern to form the mold to the required shape.The green mold is then separated from the pattern, or if the complexityof the shape is such as to require it, the mold can be hardened as bysubjecting it to carbon dioxide gas. After being separated from thepattern, the mold is baked to drive off free water at a temperatureabove the boiling point of water but not high enough to cause anexcessively violent emission of steam which could damage the mold.Following this low temperature bake, the mold is coated with a thermallyinsulative coating of a material which is nonreactive with the moltenmetal. The mold is now ready for use or it can be stored underconditions which prevent its picking up free moisture until it is to beused. Casting in molds prepared in accordance with the present inventioncan be carried out employing conventional foundry techniques. in thecasting of titanium and its alloys, centrifugal casting is preferablyused.

The refractory material used in preparing the ramming mixture can begranular graphite, mixtures of graphite and a high fusing point sandsuch as olivine sand, with the graphite forming as little as a minorportion by weight thereof, down to percent or less, or it iscontemplated that the graphite could be omitted entirely and the moldformed from the olivine sand and binder alone. Hitherto, molds 'formedof graphite tended to chill metals such as titanium and its alloys(which are characterized by a relatively narrow liquidus/solidus range).It is an important feature of this invention that the thermallyinsulating coating which is applied to baked molds in accordance withthe present invention not only greatly improves the quality of thecastings obtained from molds formed of graphite, but also makes possiblethe use of large proportions of sand or even the formation of the moldentirely from sand and binder without any graphite. While-it is not nowfully understood why this is so, it is believed that the beneficialeffect of the coating in significantly reducing surface defects in thecastings and in making possible the mold is carefully removed in thegreen state, or if the highly successful use of molds containing littlegraphite results from the effect of the coating in impeding or retardingthe transfer of heat across the mold/- metal interface and thusretarding its chilling effect.

The particle size of the refractory material is not at all critical, andsuitable particle size distributions range from about 50 to 150 inaccordance with the fineness values of the American Foundrymans Society(AFS). In the case of graphite, an AFS fineness number of about 50 to-75 may be used. Olivine sand having an AFS fineness number of about 120gave good results although a somewhat coarser and more permeable sandhaving an AFS fineness number of about 70 may also be used. I g

In forming the ramming mix, the refractory is mixed with a suitablewater-soluble binder, preferably sodium silicate which can be used inliquid or powder form. Good results have been obtained usingPhiladelphia Quartz GD sodium silicate powder. After mulling thegraphite and silicate powder, water is added and the v whole is thenmulled to provide the ramming mixture, which has a working life of about12-24 hours and preferably is used while fresh.

.Electric furnace graphite powder of the desired fineness and with aminimum of ash content has proven to be suitable. For example, SuperiorGraphite Co. grade 5018 with 2 percent or less ash content or UnionCarbide BB5 can be used. After mulling, the mixture of refractory andbinder is ready to be rammed into a flask onto the pattern. Once themold material has been rammed onto the pattern,

the shape is too complex, it is first hardened by gassing with carbondioxide gas. After the mold is removed from the pattern whether green orhardened, it is baked at a temperature above 212 F but not higher thanabout 290 F to complete hardening of the mold and to phase) from KaiserChemicals, Baton Rouge, La., ground to -325 mesh has been used'with goodresults. The alumina powder is mixed with a clear acrylic binder toprovide a slurry having a consistency suitable for spraying. Theproportions of about one pound alu- 'mina to one pint binder haveprovided good results.

The coating can also be applied by painting or dipping. Usually thecoatingis applied to a thickness of about 0.001 to 0.005 inch. Asuitable acrylic binder has been found to be polyethyl or polymethylmethacrylate in a just above the boiling .point of water, and about 250F has proven convenient.

Molds produced in accordance with the present invention provide castingswhich are characterized by greatly improved freedom from flowlines andother surface defects. These improvements are apparent when such highlyreactive metals as titanium and its alloys are cast into shapes in moldsformed substantially entirely of graphite as the refractory material.But even better overall results are obtainable when, in accordance withthe present invention, graphite forms 50% or less of the refractorymaterial. Reducing the graphite content seems to make possible the useof significantly less binder with the concomitant advantage of areduction in the amount of volatile material subsequently to be removedbefore the mold is coated and ready for use.

Conventional foundry techniquesare used in pouring andcasting the metalin the molds of the present invention, the molten metal being deliveredto the mold with sufficient force to expel gases from the metal, acrossthe metal/mold interface, and through the mold wall to its exteriorwhich is maintained under vacuum. Centrifugal casting has providedoutstanding results, but because of low mold baking temperature,resulting in incomplete 'removal of volatiles from the mold, about twicethe centrifugal force normally applied when using machined graphitemolds should be used. The rota tional speed to be used for best resultswill depend upon the permeability of the mold, mold size andcastingthickness, and can be readilydetermined in practice.

Because of the reactivity of molten metal such as titanium, a very thinsurface layer of the casting will pick up some oxygen and carbon, but,when desired, such contaminants can be readily removed by abrasiveblasting or by means of a chemical pickling bath. The depth of thecontaminated layer may range from about.0.003 to 0.010 inch,'dependingupon the thickness of the casting. I

The following examples further illustrate preferred embodiments of thepresent invention.

EXAMPLE I Graphite at about AFS fineness No. 60 and powdered sodiumsilicate in the proportions of 100 parts by weight graphite and 18 ofsodium silicate were mixed in a muller vfor about 5 minutes. Then 12parts by weight water was added, and the mixture was mulled for 3minutes to provide the ramming mixture. After ramming onto the pattern,the green mold was carefully removed and then baked at about 250 F forabout 12 hours until the mold was hardened and substantially all freewater was driven off. The required gates and risers were cut in themold, and then it was spray coated to a thickness of about 0.002 incheswith a slurry made up of Acryloid No. B82 (Rohm & Haas) in ethylenedichloride as a binder and KC8 calcined alumina (Kaiser Chemicals)ground to pass a 325 mesh sieve, in the proportions of 1 pound ofalumina to 1 pint of binder.

Molds thus formed were mounted on a centrifugal casting table with theexterior thereof subjected to a vacuum of about 100 microns. The moltentitanium was poured in the usual way but with the 5-foot diameter tablerotating at about 300 rpm so as to subject the flowing metal to about a60g force. The castings obtained in this way were substantially free offlowlines and surface defects. Because of the reactivity of tita-v nium,a surface layer of the casting about 0.003 to 0.010 inch thick picked upoxygen and carbon but this was readily removed by first subjecting thecasting to a heavy grit blast and then pickling in a 3 to 5 percentaqueous hydrofluoric acid (HF).

EXAMPLE n Molds were prepared as was described in connection withExample I except that the ramming mixture was prepared as follows Partsby Weight Graphite AFS No. 60

5O Olivine AFS No. 120 Sodium Silicate l0 l0 Water 7 has been describedutilizing sodium silicate as the water-soluble binder in preparing theramming mixture, other water-soluble binders could also be used. Forexample, it is contemplated that potassium silicate could be used withsatisfactory results in making molds for casting members of relativelythin cross-sectional thickness. 1

The terms and expressions which .have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents ofthe'features shown and described or portions thereof, but it isrecognized that various modifications are possible within the scope ofthe invention claimed.

What is claimed is:

1. The method of making a mold which includesthe steps of forming amixture of (a) a refractory material selected from the group consistingof granular graphite, olivine sand and combinations thereof, (b) awatersoluble binder selected from the group consisting of sodiumsilicate and potassium silicate, and (c) water,- shaping a mold fromsaid mixture, baking said mold at a temperature above about 2l2F but nothigher than about 290F to remove free water, and coating substantiallythe entire working surface of said mold with a thermally insulativecoating about 0.00l-0.005 inch thick formed of a fine refractory powderand a binder.

2. The method of making a mold as set forth in claim 1 in which saidwater-soluble binder is sodium silicate.

3. The method of making a mold as set forth in claim 1 in which saidmixture is shaped about a pattern to form a mold, and said mold isremoved from said pattern before it is baked.

4. The method of making a mold as set forth in claim 3 in which saidmold is hardened before it is removed from said pattern.

5. The method of making a mold as set forth in claim 1 in which saidrefractory powder is calcined alumina.

6..The method of making a mold as set forth in claim 5 in which saidbinder of said coating is an acrylic binder.

7. The method of making a mold as set forth in claim 1 in which saidrefractory material is powdered graphite.

8. The method of making a mold as set forth in claim 1 in which saidrefractory material is a mixture of powdered graphite and olivine sand.

9. The method of making a mold as set forth in claim 8 in which theparticle size of said refractory material ranges from about 50 to 150AFS. v

10. The method of making a mold as set forth in claim 8 in which saidrefractory material comprises substantially equal amounts by weight ofgraphite powder and olivine sand.

11. The method of making a mold as set forth in claim 8 in which saidrefractory material comprises 3 parts by weight olivine sand for eachpart of graphite powder.

12. The method of making a mold which includes the steps of forming amixture of powdered graphite, olivine sand and sodium silicate, addingwater to and mulling said mixture, shaping a mold from said mulledmixture, baking said mold at a temperature above aboutv 2l2F but nothigher than about 290F to remove free water, and coating at leastsubstantially the entire working surface of said baked mold with athermally insulative coating about 0.00l-0.005 inch thick formed ofcalcined alumina powder and a binder.

13. The method of making a mold ,as set forth in claim 12 in which saidmulled mixture is forced about a pattern to form said mold, and saidmold is removed from said pattern prior to baking. v

14. The method of making a mold as set forth in claim 13 in which saidmold is hardened before it is removed from said pattern.

15. The method of making a mold as set forth in claim 13 in which saidinsulative coating is formed of calcined alumina powder and an acrylicbinder.

16. The method of making a mold as set forth in claim 15 in which saidmixture comprises substantially equal parts by weight of powderedgraphite and olivine sand. v

17. The method of making a mold as set forth in claim 15 in which saidmixture comprises substantially 3 parts by weight of olivine sand foreach part of graphite powder.

18. The method of making a mold'as set forth in claim 12 in which theparticle size of said mixture ranges from about 50 to 150 AFS.

19. The method of making a mold as set forth in claim 12 in which theparticle size of said powdered graphite ranges from about 50 to 75 AFS,and the particle size of said olivine sand ranges from about to 150.

20. The method of making a mold which includes the steps of forming amixture of olivine sand and sodium silicate, adding water to and mullingsaid mixture, shaping a mold from said mulled mixture, baking said moldat a temperature above about 2l2F but not higher than about 290F toremove free water, and coating at leastsubstantially the entire workingsurface of said baked mold with a thermally insulative coating about0.00l-0.005 inch thick formed of calcined alumina powder and a binder.

21. The method of making a mold which includes the steps of forming amixture of (a) a refractory material selected from the group consistingof granular graphite, olivi'ne sand and combinations thereof, (b) awatersoluble binder selected from the group consisting of sodiumsilicate and potassium silicate, and (0) water, shaping a mold from saidmixture, baking said mold at a temperature above about 2l2F but nothigher than about 290F to remove free water, and coating substantiallythe entire working surface of said mold with a thermally insulativecoating formed of a fine refractory powder and an acrylic binder.

2. The method of making a mold as set forth in claim 1 in which saidwater-soluble binder is sodium silicate.
 3. The method of making a moldas set forth in claim 1 in which said mixture is shaped about a patternto form a mold, and said mold is removed from said pattern before it isbaked.
 4. The method of making a mold as set forth in claim 3 in whichsaid mold is hardened before it is removed from said pattern.
 5. Themethod of making a mold as set forth in claim 1 in which said refractorypowder is calcined alumina.
 6. The method of making a mold as set forthin claim 5 in which the binder of said coating is an acrylic binder. 7.The method of making a mold as set forth in claim 1 in which saidrefractory material is powdered graphite.
 8. The method of making a moldas seT forth in claim 1 in which said refractory material is a mixtureof powdered graphite and olivine sand.
 9. The method of making a mold asset forth in claim 8 in which the particle size of said refractorymaterial ranges from about 50 to 150 AFS.
 10. The method of making amold as set forth in claim 8 in which said refractory material comprisessubstantially equal amounts by weight of graphite powder and olivinesand.
 11. The method of making a mold as set forth in claim 8 in whichsaid refractory material comprises 3 parts by weight olivine sand foreach part of graphite powder.
 12. The method of making a mold whichincludes the steps of forming a mixture of powdered graphite, olivinesand and sodium silicate, adding water to and mulling said mixture,shaping a mold from said mulled mixture, baking said mold at atemperature above about 212*F but not higher than about 290*F to removefree water, and coating at least substantially the entire workingsurface of said baked mold with a thermally insulative coating about0.001-0.005 inch thick formed of calcined alumina powder and a binder.13. The method of making a mold as set forth in claim 12 in which saidmulled mixture is forced about a pattern to form said mold, and saidmold is removed from said pattern prior to baking.
 14. The method ofmaking a mold as set forth in claim 13 in which said mold is hardenedbefore it is removed from said pattern.
 15. The method of making a moldas set forth in claim 13 in which said insulative coating is formed ofcalcined alumina powder and an acrylic binder.
 16. The method of makinga mold as set forth in claim 15 in which said mixture comprisessubstantially equal parts by weight of powdered graphite and olivinesand.
 17. The method of making a mold as set forth in claim 15 in whichsaid mixture comprises substantially 3 parts by weight of olivine sandfor each part of graphite powder.
 18. The method of making a mold as setforth in claim 12 in which the particle size of said mixture ranges fromabout 50 to 150 AFS.
 19. The method of making a mold as set forth inclaim 12 in which the particle size of said powdered graphite rangesfrom about 50 to 75 AFS, and the particle size of said olivine sandranges from about 70 to
 150. 20. The method of making a mold whichincludes the steps of forming a mixture of olivine sand and sodiumsilicate, adding water to and mulling said mixture, shaping a mold fromsaid mulled mixture, baking said mold at a temperature above about 212*Fbut not higher than about 290*F to remove free water, and coating atleast substantially the entire working surface of said baked mold with athermally insulative coating about 0.001-0.005 inch thick formed ofcalcined alumina powder and a binder.
 21. The method of making a moldwhich includes the steps of forming a mixture of (a) a refractorymaterial selected from the group consisting of granular graphite,olivine sand and combinations thereof, (b) a water-soluble binderselected from the group consisting of sodium silicate and potassiumsilicate, and (c) water, shaping a mold from said mixture, baking saidmold at a temperature above about 212*F but not higher than about 290*Fto remove free water, and coating substantially the entire workingsurface of said mold with a thermally insulative coating formed of afine refractory powder and an acrylic binder.